In general, if a screw is used for a longer time, the screw will round so that it is difficult to drive the screw by a spanner. Thereby, it is often that the screw must be destroyed for taking the screw out. This will destroy the work piece.
Thereby, to improve the defect in the prior art, there are some ways are developed for resolving the prior art problem.
However, all these improvements are aimed to improve outer hexagonal screws (that is, a screw has a hexagonal driving portion at an outer side of the screw) instead of improving inner hexagonal screws (that is, a screw has a hexagonal driving portion at an inner side of the screw).
This is because, referring to FIG. 9, the conventional spanner for driving an inner hexagonal screw has a solid structure which does not deform. Each spanner is designed for screws with predetermined sizes. As the inner hexagonal screw has rounded edges so that as the hexagonal screw is driven, it will slide with respect to the spanner so that it can not be effectively driven.
However, for the outer hexagonal screws, the conventional spanner has expandable clamping portions at two ends thereof so that as the edges of the hexagonal portion of an outer hexagonal screw are rounded, the clamping portions can deform to match the round edges of the outer hexagonal screws, but no this mechanism is design for the conventioanl spanner for driving the inner hexagonal screws.